Temperature measuring techniques providing readouts in the cryogenic range are finding broadened applications across the scientific spectrum. These applications may range from superconductivity to devices intended for location in outer space. Generally, the type thermometer involved in such measurements is constructed employing a sensing component which has a capability to change one or more of its sensible electrical parameters or characteristics as the temperature of its environment is changed. Sensing elements which exhibit these characteristics are provided in a variety of forms, including thermocouples, resistive elements and, more recently, solid-state devices such as diodes. Selection of a particular sensing element generally involves trade-offs and usually will be predicated upon the intended use of the device. As an example, a wide range sensor is desirable for applications such as high vacuum systems which are heated or baked to temperatures of at least 250.degree. C. such that their thus-energized surfaces are outgassed. Subsequently, the same systems are operated in the cryogenic range, often at temperatures below 4.degree. K. Where a thermometer is to be mounted within such high vacuum systems, it is called upon to survive the baking procedures and yet remain stable to achieve accurate readings at the lower cryogenic range temperatures. One device recently made available for this application is a Rh-Fe resistance thermometer. However, its applications are limited by cost, size, and other factors.
Resistive devices provide readouts within the cryogenic range, but are often bulky and perform only in conjunction with more elaborate electrical support and with more sophisticated measurement methods. Conversely, solid-state devices, such as diodes, are relatively facile to operate, a constant current source being employed with them and the forward junction voltage drop (V) being utilized as the temperature related readout. Silicon diodes are popularly employed as thermometers in the cryogenic range, however, their use is subject to operational restrictions. For example, it is desirable that the sensitivities of the diodes (dV/dT, where T=Temperature), exhibit a smooth transition over their operational temperature ranges. Where such smooth transitioning is available, polynomial curve fits are achieved readily by the practitioner. Unfortunately, the silicon devices display somewhat complex sensitivity characteristics. Many applications of thermometry also involve environments wherein the sensing elements are subjected to magnetic fields of varying intensities. In part because of the forbidden band characteristics of certain of the diode structures, they will exhibit an unwanted shift of output under magnetic environments. Further, each previously available diode material has traditionally exhibited somewhat limited capabilities for measuring both at elevated temperatures as well as in the cryogenic range. Another aspect considered in the selection of thermometer sensing elements resides in the repeatability of their performance. While these elements may exhibit a requisite sensitivity over desired temperature ranges, where their readouts vary or drift over a number of temperature range cycles, their suitability for employment as thermometers is negated. Thus, the selection of any element for thermometry involves subjecting the materials to repeated cycling, for example, from room temperature to that of liquid helium and a subsequent evaluation of any drift in their output from cycle to cycle. Still another aspect in the selection of thermometer components resides in their sensitivity at very low cryogenic ranges, for example below 4.degree. K. Very minor alterations of temperature at these levels translate into substantial percentages of temperature change. Thus, lowered sensitivities at these critical ranges are unfortunate.
For many applications, another aspect for sensor selection resides in their capability for "matching". Matched sensors will exhibit very similar voltage-temperature characteristic curves. Where such characteristic standardization is available, calibration procedures may be avoided.
From the foregoing, it may be observed that desired characteristics in the selection of sensing elements for cryogenic range thermometers includes the facileness with which they may be used, as exhibited in solid-state diode structures; the stability of such devices in providing accurate readouts over numerous cycles of use; the relative immunity of such devices to the effects of magnetic fields and the like; the capability of such devices to be useable over a wide temperature range; and the smooth transitioning of sensitivity curves which such devices may exhibit, affecting the ease of calibration curve fitting.